The debarking process is a very important step for many industrial uses of wood from logs. Wood chips produced through the debarking process are the main raw materials of most pulp and paper, MDF and HDF boards industries. Bark has minimal value and may be associated with net financial loss incurred by the forest industry. Wood chips typically come from sawmills where they are produced from logs that have been debarked, and the debarking quality directly influences the chip quality and sawmill production yield. The debarking of wooden logs is an operation consisting of removing from the surface of each log a thin layer of material mainly containing bark (outer bark and inner bark) with some cambium, with the objective to preserve intact the fiber material under the cambium layer, so as to maximize economic yield obtained from lumber and wood chips. Typically, wood chips represent roughly 50% of the volume of material produced, within which a maximum of 1% (1.5% in winter) of bark content is targeted, while aiming at preserving the full potential of lumber yield for each piece of timber. Since bark remaining on chips deteriorates the pulp being produced, it is within the industry practices that buyers of wood chips such as pulp producers and pressed board manufacturers, to impose economic penalties to chips providers or even refuse delivery in cases where bark content limit is exceeded. In the other hand, the compliance with that requirement may involve fiber loss from log surface as a result of debarking, causing a loss of income, due to fiber material loss in wood chips associated with residual fiber in bark, as well as to the lower value of lumber production attributed to a lower quality associated with surface and dimensional defects. In other words, residual bark on the debarked log surface increases the percentage of bark content in the wood chips which may thus be downgraded or refused by the mills, while excessive debarking will result in fiber loss, thus decreasing sawmills revenues. The optimization of debarking process is very complex, mainly due to numerous variables to be considered in order to concurrently decrease fiber loss and residual bark on the debarked log surface. In practice, seeking to operate the debarking process at an optimal point is a difficult task. Generally, wood processing involves a large variation in raw material characteristics at the input end of the sawing process in contrast with upstream processes such as drying and planning. Therefore, debarking should be entirely efficient in terms of production capacity and debarked log quality while being fed in timber pieces of various diameters, moisture contents and wood species. The debarking equipment currently available includes the drum debarker, hydraulic debarker and rotary ring debarker, the two latter being used on sequential, respectively transverse and lengthwise, log processing lines, while the former is used for debarking logs in batches. During the past years, the rotary ring debarker has been the most used type of debarking equipment in sawmills. The rotary ring debarker makes use of spinning knifes mounted on a rotary ring as cutting tools, whose angular position as well as the magnitude of controlled pressure applied thereto are usually adjusted periodically to achieve an acceptable debarking quality taking into account a large variation at the input of the debarking process.
The variations of log physical properties, environmental and storage conditions, as well as debarking operating conditions strongly influence the residual bark and fiber loss levels on the surface of debarked logs. The performance of a rotary ring debarker depends on log characteristics such as wood species, moisture content, freshness, mean diameter, and bark thickness. In particular, different wood species have their own characteristics that may affect the debarking process, for example black spruce is more easily debarked than balsam fir during winter. Moisture content is the most important factor in wood/bark adhesion strength. As to freshness, lowest bark/wood shear strengths can be observed with fresher logs as compared to aged logs. As to diameter, it is generally observed that amongst the debarked logs of a processed batch, those of small diameter are often poorly debarked. It is also well known that bark thickness varies between wood species, parts of the tree, as well as harvesting areas, and thus influences the choice of debarking operating parameters, especially pressure applied to the tools, as reported by Spurr, S. H, et al. in “Forest Inventory” The Ronald Press Company, NY. 1952, pp. 476; Philip, M. S. in “Measuring Trees and Forests”, 2nd ed. CAB Inter., Wallingford, Oxforshire, UK. 1994, pp. 310; Wilhelmsson, L., et al. in “Models for predicting wood properties in stems of Picea abies and Pinus sylvestris in Sweden”, Scand. J. Forest Res. 17, 2002, pp. 330-350; and Marshall, H. D., et al. in “Effects of bark thickness estimates on optimal log merchandiding”, Forest Products Journal Vol. 56, No. 11/12, 2006. As to the effects of environmental and storage conditions on debarking quality, air temperature, which influences temperature state of log surface (i.e. frozen or non-frozen), has an effect on wood/bark adhesion strength when the log moisture content is greater than fiber saturation point, as reported by Baroth, R., in “Literature review of the latest development of wood debarking”, Control Engineering Laboratory, University Oulu, Report A No. 27, August 2005, p. 4. The storage conditions such as storage time, ambient air conditions (temperature, relative humidity, velocity) also influence moisture content distribution in logs, which results in variation of wood/bark adhesion strength finally giving rise to debarking quality. Finally, the performance of a rotary ring debarker depends on its main operating parameters, namely pressure applied to the cutting tools, log feed speed, ring rotational speed, and cutting tool tip overlap. It is known that a poorly maintained or misadjusted debarker may cause log surface damages and well as sections of reduced diameter along the debarked log, which influences log classification and optimization by reducing available wood for lumber production. As to pressure radially applied to cutting tools, values for that parameter may be assigned according to log physical characteristics as well as environmental and storage conditions. At sawmill, pressure values are typically determined according to wood species, average log diameter and log surface log temperature state (frozen or non-frozen), and the radial pressure may be adjusted during debarking as the debarking log diameter is changed. When the radial pressure is set at low value to prevent fiber loss, more residual bark is thus observed on the debarked log surface. Log feed speed is considered as an important factor of debarking yield, and that parameter is usually kept at a high value at sawmill. As to ring rotational speed, a high value assigned to that parameter increases tool arms centrifugal forces and decreases tool reaction time, which factors are beneficial to debarking yield increases as reported by Laganiere, B. et al. in “Effects of radial force and tip path overlap on the ring debarking efficiency of frozen balsam fir logs”, Forest Products Journal, vol. 55, No. 3. 2005. As a result of combined effects of log feed and ring rotation, consecutive debarking tool tip path overlap occurs during debarking, and to ensure complete bark coverage by the tips, an overlap factor over 10% has been suggested by Lapointe, J. A. in “Optimizing the operation of ring debarkers. Research Memorandum”, Project No. 76-0207-01. Domtar Research Center, Senneville, Qc, Canada. 1976, pp. 45. For a given value of log feed or ring rotational speed, the tool tip path overlap can be increased by decreasing the ring rotational or feed speed, respectively.
Generally, operation of the rotary ring debarker is based on experience of sawmill operators, the operation parameters such as pressure applied to the cutting tools, log feed speed and ring rotational speed being set up for different wood species and log temperature states (frozen or non-frozen). For years, the quality control at the output end of known debarking equipment has been generally limited to visual control and classification by an operator, which is not precise and results in a relatively high classification error rate. In some lumber mills, samples are periodically selected in order to determine the amount of bark into wood chips or the amount of fiber present into bark. These procedures give merely rough indications of debarker performance that merely allow trend monitoring, without providing accurate quality control. For years, many automated system have been proposed to automate debarking quality detection generally based on sensors measuring surface reflectivity of the debarked logs or samples thereof, such as disclosed in the following patent publications: U.S. Pat. Nos. 2,769,468; 5,247,978; 5,274,244; 5,335,790; 6,137,894; 6,166,393; 6,493,076; 6,526,154; 6,614,041; 6,539,993; and US 2010/236664. However, surface reflectivity alone cannot discriminate all of the various characteristics associated with debarked surface quality to allow an accurate control thereof. More recently, the use of three-dimensional profile data for detecting surface defects on debarked logs has been proposed by L. Thomas in “Automated detection of surface defects on barked hardwood logs and stems using 3-D laser scanned data” Virginia Polytechnic Institute and State University, Virginia, U.S.A., September 2006. However, the contour-based detection approach as taught by Thomas is limited to the identification of defects characterized by significant height changes, such as defective knobs and depressions, the detection of which defects cannot allow an accurate control over the quality of debarking.